I am refering to page 306, figure 14.18c with the voltage across each junction deduced from the values shown at left.

Currently, on figure 14.18c, the voltage across R1 is about an 1.4 V determining a current about 0.7 mA in R1 and R2. It is indicated as being 1 mA. With 0.7 mA, the calculated bias spreader votage is about 3.6 V

Decreasing R1 from 2200 Ohm to 1300 Ohm just as it is in figure 14.19a, this should give a current of 1 mA and a bias spreader voltage as 4.2 V, as written.

I am refering to page 306, figure 14.18c with the voltage across each junction deduced from the values shown at left.

Currently, on figure 14.18c, the voltage across R1 is about an 1.4 V determining a current about 0.7 mA in R1 and R2. It is indicated as being 1 mA. With 0.7 mA, the calculated bias spreader votage is about 3.6 V

Decreasing R1 from 2200 Ohm to 1300 Ohm just as it is in figure 14.19a, this should give a current of 1 mA and a bias spreader voltage as 4.2 V, as written.

Regards.

Hi forr,

Yes, it looks like you are right. Good catch. I deeply appreciate any errors in the book being brought to my attention.

I have been having a hard time putting together a bias spreader that could correctly track the 2.1mV/C NJL Vbe with the 1.7mV/C ThermoTrak diode tempco for a triple output stage that has a diamond buffer as its pre-driver and driver, which requires a bias voltage of about 1.4V. This 1.4V really is a very small play ground that doesn't house many p/n junction drops. So after some head scratching I came up with a circuit that seems to be able to serve the purpose.

The circuit diagram is in the attached pdf. It uses an op amp MAX4289 that works on as low as 1.0V supply and swings rail-to-rail. I set it up to have a gain of 2.5 to derive a 4.25mV/C for the two b-e junctions of the output transistors from the TT diode's 1.7mV/C tempco. R6 is the bias trim pot. It sets a bias current in the TT diodes so as for them to come up with a proper forward drop. R5 may serve as a supplement adjustment just in case. I could have used only one of the TT diodes to make the circuit simpler but I would feel better by using one each from the NPN and PNP devices.

I must admit I have limited experience in circuit design and engineering. Before I put it into PCB layout and make smoke in my amp I would much appreciate any of your comments and ideas about it or any other 1.4V bias spreader circuit that may track correctly with TT diodes.

By the way, Bob you wrote one awesome book! It connects many isolated "dots" and it's a pleasure to read. Thank you so much!

Wonderful information.... you highlight the static and dynamic tempco, and suggest it's best done empirically.

indeed

Hi

I find that since the bias for vertical mosfets is usually >150mA, allowing the system to be a bit overcompensating can buy a bit of extra stability with regard to dynamic tempco. Although the effect is small, the fets get a boost in Gm at a higher temperature. If the effect of overcompensation is slight, the bias can be kept within acceptable range, >100mA at maximum temperatures. Consequently, initial static bias may be a bit higher but since the devices and heatsink are cold it is of slight issue. I have had success in stablizing bias of vertical mosfets using a topology similar to the HEC Bob used, but I use small SMD devices as the error amplifiers, mounted on the PCB so as to be in contact with the drain pin close to the transistor package. Although there is a short distance of metal for the heat to traverse, it is not so effected by the time lag of the thermal insulator/heatsink. see pic from previous learning experience.

I have been having a hard time putting together a bias spreader that could correctly track the 2.1mV/C NJL Vbe with the 1.7mV/C ThermoTrak diode tempco for a triple output stage that has a diamond buffer as its pre-driver and driver, which requires a bias voltage of about 1.4V. This 1.4V really is a very small play ground that doesn't house many p/n junction drops. So after some head scratching I came up with a circuit that seems to be able to serve the purpose.

The circuit diagram is in the attached pdf. It uses an op amp MAX4289 that works on as low as 1.0V supply and swings rail-to-rail. I set it up to have a gain of 2.5 to derive a 4.25mV/C for the two b-e junctions of the output transistors from the TT diode's 1.7mV/C tempco. R6 is the bias trim pot. It sets a bias current in the TT diodes so as for them to come up with a proper forward drop. R5 may serve as a supplement adjustment just in case. I could have used only one of the TT diodes to make the circuit simpler but I would feel better by using one each from the NPN and PNP devices.

I must admit I have limited experience in circuit design and engineering. Before I put it into PCB layout and make smoke in my amp I would much appreciate any of your comments and ideas about it or any other 1.4V bias spreader circuit that may track correctly with TT diodes.

By the way, Bob you wrote one awesome book! It connects many isolated "dots" and it's a pleasure to read. Thank you so much!

Hi Nattawa,

As Edmond indicated, it looks like that circuit you provided may work, and it appears quite clever. Have you SPICE'd it yet?

You are right about the bias compensation of the Diamond Buffer Triple (DBT) output stage. The good news is that the bias tempcos of the predriver and driver tend to cancel each other out. The bad news is that the only Vbes left that need the spread are those of the output transistors. As you pointed out, this does not give you a lot of voltage "room" to play with. I agree that one is wise to use one each of the P and N ThermalTrak diodes to form the bias spreader.

I do tend to like simpler more "passive" bias spreaders (less to go wrong) and there may be some other approaches that are effective. As we know, if there is a greater bias spread needed, we have more circuit flexibility in achieving the bias speader TC that we desire (see the ThermalTrack bias spreaders I showed for more conventional output stages, like Triples, in Figures 14.18 and 14.19).

One approach that I showed in the book is to use a Diamond Buffer Quad (DBQ) like I showed in Figure 25.7. This introduces a larger driver AFTER the diamond buffer, sometimes one as big as an output transistor. This is especially useful for really high power amplifiers, but is certainly practical for any power level. It now makes the bias spreader need to supply 4 Vbe and gives you more room to play and adjust tempco. The DBQ is compensated in a way like a Darlington output stage.

Another approach that can be used with the DBT to get more bias spreader headroom is to add a pair of small diodes in series with the resistor that sets the bias current of the diamond buffer. The output transistor bases are then fed from across the current-setting resistor.

One can also make a higher-voltage bias spreader and reduce its spread right at its output with a three-resistor attenuator whose inner nodes feed the bases of the DBT. The current through this resistive attenuator should be held to less than about half the total current flowing through the total spreader circuit. The necessary drops could also be obtained with diodes instead of the outer attenuator resistors. This approach does not require any additional transistors to see high voltage, as might be the case if using the DBQ.

As you know, the key to all of this is to be able to amplify the TC of the pair of TT diodes enough to achieve the needed overall TC to match the output transistor Vbes. Lots of interesting approaches are possible, and the one you showed appears to be workable and clever.

@ Bob, thanks again for your comments and great many ideas about the alternatives. I think I share the philosophy behind your preference of using "passive" parts where possible. My op-amp based biasing circuit is really a lazy man's quick solution.

@ Edmond, thanks for the links to the discussions about your Phoenix project. When I first saw the schematic, it reminded me of my first view of the great Milan Cathedral. It's very good that you brought up the thermal attenuation. I haven't gone that far into Bob's book to arrive at that part. I may have to consider and compensate for it in an actual design.

Check the datasheet, MAX4289 has only 17KHz GBW. Meaning that @17KHz there won't be any loop gain to keep the bias constant.

Hi Waly,

I thought the bias spreader is just a floating piece of DC battery, and I thought the 17KHz GBW is way more than sufficient for any rate of temperature change over time a TT diode could ever see.... did I miss something here?

I thought the bias spreader is just a floating piece of DC battery, and I thought the 17KHz GBW is way more than sufficient for any rate of temperature change over time a TT diode could ever see.... did I miss something here?

Nope, the impedance of the bias spreader has to be low even and high frequencies, read Bob's book. You can always add a cap across the spreader, but that cap comes with other penalties in terms of distortions.